In recent years one has become increasingly aware of the impact of human activities on the environment and the negative consequences this may have. Ways to reduce, reuse and recycle resources are becoming more important. In particular, clean water is becoming a scarce commodity. Therefore, various methods and devices for purifying water have been published.
A method for water purification is by capacitive deionisation, using an apparatus provided with a flow through capacitor (FTC) for removal of ions in water. The FTC functions as an electrically regenerable cell for capacitive deionisation. By charging electrodes, ions are removed from an electrolyte and are held in electric double layers at the electrodes. The electrodes can be (partially) electrically regenerated to desorb such previously removed ions without adding chemicals.
The apparatus for removal of ions comprises one or more pairs of spaced apart electrodes (a cathode and an anode) and a spacer, separating the electrodes and allowing water to flow between the electrodes. The electrodes are provided with current collectors or backing layers and a high surface area material, such as e.g. carbon, which may be used to store removed ions. The current collectors may be in direct contact with the high surface area material. Current collectors are electrically conductive and transport charge in and out of the electrodes and into the high surface area material.
A charge barrier may be placed adjacent to an electrode of the flow-through capacitor. The term charge barrier refers to a layer of material which is permeable or semi-permeable for ions and is capable of holding an electric charge. Ions with opposite charge as the charge barrier charge can pass the charge barrier material, whereas ions of similar charge as the charge of the charge barrier cannot pass the charge barrier material. Ions of similar charge as the charge barrier material are therefore contained or trapped either in e.g. the electrode compartment and/or in the spacer compartment. The charge barrier may comprise an ion exchange material provided in a membrane. A membrane provided with ion exchange material may allow an increase in ionic efficiency, which in turn allows energy efficient ion removal.
FIG. 1a gives a schematic representation of the charging of the carbon coated current collector during the ion removal step. During ion removal anion 1 pas the anion exchange membrane 3 (charge barrier) and enter into the carbon electrode (the first carbon coated current collector) 5. These ions are mainly stored in the electrical double layers that form at the carbon-water interface upon electrically charging of the electrode 5. In this example the anions 1 can pass the membrane 3, whereas the cations 7 cannot. The cations 7 are expelled from the carbon-water interface, but cannot pass the membrane 3 and are therefore accumulated inside the electrode pores.
FIG. 1b gives a schematic representation of the discharging of the carbon coated current collector during the electrode regeneration step at reversed potential. During electrode regeneration at reversed potential, the electrode 5 is now negatively charged and the countercharge therefore consists mainly of cations 7. These cations are removed from the carbon pores and are accumulated at the carbon-water interface. As a consequence the salt concentration in the carbon pores becomes very low, because the cations cannot enter the electrodes, whereas the anions are removed from the electrode space during regeneration. This low salt concentration may lead to an increased resistance during regeneration, especially when the first charge barrier layer 3 is not in intimate contact with the first carbon coated current collector 5.
The functioning of the charge barrier in the flow through capacitor is not always optimal because the carbon coated first current collector may have a roughness which may cause the first charge barrier to have a less optimal contact with the carbon coated first current collector.